1. Technical Field
This invention relates generally to a semiconductor package, and more particularly to an inner lead bonding apparatus comprising a heat dissipation means for preventing heat from being transferred to the tape automated bonding tape during inner lead bonding process, and to a method for inner lead bonding using such an inner lead bonding apparatus.
2. Description of the Prior Art
Tape automated bonding thereinafter "TAB" which was introduced by General Electric Co. in 1960's is one of automated technologies for packaging a plurality of semiconductor devices in place of wire bonding technology. As TAB technology develops and the reliability of TAB increases, the application of TAB packages are gradually broadened to the fields requiring more stable and more excellent electrical properties such as Very High Speed ICs, Liquid Crystal Displays, Super Computers or and the like.
TAB process begins with the step of bonding, by using a thermo-compression bonding technique, a silicon chip to a patterned metal, for example, a copper pattern formed on a polymer tape (e.g., polyimide tape). Generally the tape used in TAB comprises an adhesive layer made of various adhesive materials including polyimides, epoxies, acrylics and phenolic-butyrals. The choice of an adhesive should be made based, with first priority, on its thermal stability, because a TAB package will be challenged during elevated temperature processes such as Inner Lead Bonding (here-in-after, referred to as `ILB`), encapsulation-curing, burn-in testing and outer lead bonding. In particular, an ILB process in which patterned inner leads are bonded to bumps formed on bonding pads of the semiconductor chips is carried out under a condition of 530.degree.-550.degree. C. in order to increase the pull strength of the junction interface between inner leads and the bumps. The ILB process is typically accomplished by either a single point bonding method or a gang bonding method. The gang bonding, method which is disclosed in U.S. Pat. No. 3,763,404 and U.S. Pat. No. 4,051,508, is widely used in mass production of TAB packages in a short processing cycle since TAB beam leads can be bonded simultaneously by methods including thermo-compression bonding, dynamic alloy formation, solder reflow, and the like.
FIG. 1 shows a prior art TAB package during ILB process is running. Referring to this Figure, inner leads 1 formed in a predetermined pattern by, for example, photolithography is attached by an adhesive 2 to a polymer (e.g., polyimide) layer 3 to form a three-layered tape. On the chip bonding pads 7 of a semiconductor device 8 are formed bumps 6 for electrical interconnection with the inner leads 1. In ILB, a plurality of semiconductor chips 8 are automatically pre-aligned by an xy-coordinate table (not shown) with reference to a bonding machine 5 such as a thermode. A supporting means or clamp 4 is used to fasten a lead frame of TAB, and the position of the former is preset, optimized so as to secure a stable fixation of the TAB leads. During the ILB process, a chip carrier (not shown) rises to the bond level, and the bonding machine 5 drops to apply heat and pressure through metallic leads 1 on the tape to the bumps 6 on the chip 8. At this time, the heat conducted to the adhesive 2 of the lead frame from the bonding machine 5 is likely cause a so called degradation phenomenon in which the thermally fragile adhesive is melted down. Even if this does not occur, high stress is inevitably put on the interface of the adhesive, which and the leads, because the adhesive expanded during ILB process contracts after ILB is finished. This kind of stress can be also applied to the junction point between the inner leads 1 and the bumps 6 because of the difference in their thermal expansion coefficients. This will result in breaking-off the ILB junction interface which will, in turn, cause critical electrical failures of the device.
The dwell time taken by the thermode 5 in thermo-compressing the leads just lying on the bumps, and the temperature of the thermode 5 have a very important influence upon the bonding strength of the ILB junction interface. This strength will be apparently increased in proportion to the temperature and the dwell time. However, the problems of stress, breaking-off of the interface and the melting down of the adhesive as described above will be likewise made more severe. Accordingly, a compromise of the two factors must be made.